Speed of motorboat in still water is 45 km/h. If the motor travels 80 km along the stream in 1 h 20 min, then the time taken by it to cover the same distance against the stream will be?

Let speed of stream be x km/h
Given speed of motorboat in still water = 45 km/h
∴ Speed of boat along stream = (45 + x) km/h
According to the question,
45 + x = 80 x 1¹/₃
⇒ 45 + x = 80 x 3 / 4
⇒ x = 60 - 45 = 15
⇒ Speed of boat against stream = 45 - 15 = 30 km/h
Hence, required time = Distance / Speed = 80 / 30
= (8 x 60)/3 = 160 min
= 2 h 40 min